I've had both cases in question and I would say Stacker 830 all the way. The Stacker 830 is an over-built, battle cruiser of a case. It's all aluminum and has a removable motherboard tray which is actually pretty nice to have. A side-effect of this is having the ability to mount standard ATX or reverse ATX (Lian Li style) It looks ridiculous without its front door IMO so hopefully you don't mind having a door on your case. It will take any PSU you throw at it (830 Evo, 832, 830 Nvidia)
Cosmos S is nice to but with the side fan you're going to be limited in terms of coolers you can use. It's main advantage in this comparison is the ability to mount a triple rad and fans on the roof without modding. Plenty of room for pump and res inside as well. It has better cable management options than the Stacker but I didn't find there was a lot of room behind the motherboard tray anyway and ended up routing my 24-pin normally. I felt the Cosmos was solid but didn't have the same build quality of the Stacker. The front bezels seemed flimsy and Cooler Master used a lot of plastic on it.
The Stacker can be effectively watercooled. Here are a couple of mine:
Using a radbox and an external 240
This also is a straight bolt-on. There's a vent in the floor meant to provide air to the lower card when using SLI. It just so happens the bolts line up perfectly for a 240 rad.